The invention relates to a multi-cylinder internal-combustion engine and to a method of operating such an internal-combustion engine.
An effective measure for lowering nitrogen oxide emissions of internal-combust on engines consists of recirculating the exhaust gas. This is achieved by reducing the oxygen partial pressure in the fed fresh gas. Such a system is known, for example, from European Patent Document EP 0 442 981 B1 (corresponding U.S. Pat. No. 5,121,734). In the case of the internal-combustion engine illustrated there, the exhaust gas of an input cylinder is fed into a common intake manifold. A mixture of intake air and recirculated exhaust gas is therefore fed to all cylinders.
From the applicant""s German Patent Document DE 43 31 509 C1 (corresponding U.S. Pat. No. 5,517,976), a divided engine with an exhaust gas recirculating system is also known. The divided engine consists of output cylinders, in the following called xe2x80x9cfirst cylindersxe2x80x9d, and of at least one input cylinder, in the following called xe2x80x9csecond cylinderxe2x80x9d. In this case, the intake system is divided into two parts. The first cylinders receive a mixture of intake air and recirculated exhaust gas of the second cylinder. The second cylinder receives only intake air.
Based on the above-described prior art, it is an object of as the invention to further develop this prior art with respect to a further lowering of the nitrogen oxide emissions.
This object is achieved, on the one hand, by a method of operating a multi-cylinder internal-combustion engine and, on the other hand, by a multi-cylinder internal-combustion engine having certain characteristics, as set forth below:
(i) Method of operating an internal-combustion engine which has first cylinders and at least one second cylinder, the second cylinder serving as an input cylinder, in that a portion of the exhaust gas is fed by way of an exhaust gas recirculating system to the carbureted fuel, and having an injection system for injecting fuel into the first cylinders and the second cylinder of the internal combustion engine, characterized in that the controlling of the injected fuel quantity of the second cylinder takes place independently of the controlling of the injected fuel quantity of the first cylinders.
(ii) Internal-combustion engine which has first cylinders and at least one second cylinder, which second cylinder is used as an input cylinder in that a portion of the exhaust gas is fed to the carbureted gas by way of an exhaust gas recirculating system characterized in that an injection system is provided for injecting fuel into the first cylinders and into the second cylinder of the internal-combustion engine, the injection system permitting a mutually independent controlling of the injected fuel quantity of the first cylinders and of the second cylinder.
The method according to the invention provides for an internal-combustion engine of the above-mentioned type where the controlling of the injected fuel quantity of the second cylinder is independent of the controlling of the injected fuel quantity of the first cylinders.
It is an important advantage of he method according to the invention that, as a result of the independent control, a lowering of the nitrogen oxide emissions is permitted over a wide operating range, in which case the internal-combustion engine can simultaneously be operated in an optimal manner with respect to its power yield.
As a further development of the invention, it is suggested that the injection start and/or the injection duration of the fuel injection for the first cylinders and the second cylinder can be adjusted independently of one another.
For this purpose, the fuel quantity injected in the second cylinder is reduced for raising the partial oxygen pressure of the carbureted fuel fed to the first cylinders. Simultaneously, together with the reduction of the fuel quantity injected in the second cylinder, the fuel quantity injected in the first cylinders can be raised. The advantage consists of the fact that the reduction of the fuel usage for the input cylinder (second cylinder) is not, as in the case of a conventional exhaust gas recirculation with a reduction of the fuel usage for all cylinders, connected with a considerable reduction of the maximum combustion pressure. This is particularly advantageous in the case of supercharged engines, in which the reduction of the fuel usage at the few input cylinders influences the supercharging pressure only slightly and by the raising of the fuel quantity injected in the first cylinders (output cylinders), the total output of the internal-combustion engine is maintained. Another advantage consists of the fact that the stressing of the crankshaft of the internal-combustion engine increases only little as a result of the non-uniform loading because of different maximum combustion pressures in the cylinders.
In another embodiment, it is provided that, for lowering the partial oxygen pressure of the carbureted fuel fed to the first cylinders, the fuel quantity injected in the second cylinder is increased. And, as a further development of the latter, it is provided that, together with the raising of the fuel quantity injected in the second cylinder, the start of the injection of the fuel quantity into the second cylinder is shifted to late. A raising of the maximum combustion pressure of the input cylinder (second cylinder) can therefore be avoided despite the increase of the injected fuel quantity.
According to an advantageous further development of the process according to the invention, it is provided that, during acceleration operations of the internal-combustion engine, the fuel quantity injected in the second cylinder is reduced or the injection of fuel into the second cylinder is completely switched off. As a result, the soot formation during acceleration operations will not be additionally intensified by the exhaust gas recirculation.
According to another advantageous further development of the process according to the invention, it is provided that, in the idling operation of the internal-combustion engine, the fuel injection into the second cylinder is switched off. As a result, it is possible to avoid a sooting in the idling operation because of an undercooling of the exhaust gas flow below the dew point in the recirculation pipe train.
According to certain preferred embodiments of the invention, the controlling of the partial oxygen pressure of the carbureted fuel fed to the first cylinders takes place as a function of one or several of the following values in the characteristic diagram of the internal-combustion engine operation: Cylinder pressure, concentration of exhaust gas constituents, particularly NOx, HC, CO, exhaust gas temperature, engine torque, fuel usage, supercharging pressure, rotational engine speed.
For implementing the method according to the invention, it is suggested according to certain preferred embodiments of the invention that the injection of the fuel takes place by means of a common rail injection system. This injection system has a common preliminary storage device for storing pressurized fuel. The common preliminary storage device is connected by way of injection pipes with fuel injectors for injecting the fuel into the cylinders of the internal-combustion engine. For controlling the fed fuel quantity, a control unit is provided.
As an alternative thereto, an injection system may also be provided in which an individual storage device is assigned to each fuel injector. This fuel injector, in turn, is supplied with fuel by way of a distributor pipe by a common supply line. Also in this case, the controlling so the injection is carried out by the control unit.
The invention will be explained by means of the illustrated embodiments.